Some plate-like members used in transport airplanes, construction structures, and the like are fastened to other members with bolts. FIG. 1 is a plan view illustrating an example of a plate-like member having a plurality of fastening portions. This plate-like member 1 has a substantially flat circular plate shape. The plate-like member 1 includes three fastening portions 2 in a region spaced apart from the margin. The plate-like member 1 is bolted to another member at the fastening portions 2.
When the surface of the plate-like member 1 serves as the bearing surface of the bolts, the fastening strength is influenced by the thickness around the fastening portions 2 of the plate-like member 1 to a large extent. When the fastening strength is required to be increased, it is necessary to either increase the thickness of the plate-like member 1 or to attach a reinforcing plate to the fastening portions 2.
However, when the thickness of the whole of the plate-like member is increased, the weight of the plate-like member markedly increases. When a product including the plate-like member is demanded to be decreased in weight, such demand cannot be satisfied. Furthermore, the use of the reinforcing plate increases the number of components, thereby raising the costs of management and assembling. To address these concerns, it is conceivable that only a necessary portion (in the example of FIG. 1, the fastening portions 2 and portions around the fastening portions 2) of the plate-like member is increased in thickness (hereinafter, also referred to as “thickened”).
Patent Literature 1 discloses a thickening press process including: a previous step of deep-drawing a work; and a step of pressing the deep-drawn work to thicken a predetermined portion. Patent Literature 2 discloses a method of clamping a pressed work having an end wall and a bent portion that is bent from the outer circumference toward one side, at the bent portion from the outer circumference of this end wall, so that the bent portion is compressed in the radial direction for thickening. Patent Literature 3 discloses a method of manufacturing a molded component having a bent corner, including a thickening step of increasing in thickness of a corner by molding under pressure.